In today's tape libraries, a robotic accessor is used to move tape cartridges from storage bins within the library to tape drives where the cartridge can be read or written. These robotic accessors require power to provide the motion and logic onboard the accessor and also require communication with the rest of the library system in order to perform their most basic tasks. Typically, the power and communication are provided to the accessor through a dynamic cable that is tethered between the accessor and some static portion of the library chassis or frame. Today, many libraries are scalable so that expansion frames or other units can be added to the base frame or unit. During expansion the dynamic cable that is attached to the accessor must be replaced so that it can adapt to this new length of accessor travel. This dynamic cable replacement is a costly service replacement and an unwanted customer outage where the robotic accessor becomes unavailable while the dynamic cable is being changed. Also, by having cables of different lengths to accommodate the different modular frame sizes, a costly inventory of different cable lengths is required in manufacturing sites and service replacement part centers. Additionally, some customers would like more robotic performance in their large library systems and multiple robotic accessors could provide this performance.
If each accessor is tethered with a cable, the cables between multiple (more than two) accessors could quickly become tangled and cause the system to fail. It is also desirable to configure large libraries with a nonlinear accessor path. While cables can be easily designed to roll and unroll reliably in a linear fashion, for an accessor to turn a corner or move in a different nonlinear direction, it becomes very difficult to have the cable follow the accessor in its nonlinear path. Due to these difficulties, a robotic accessor without the cable is desirable.
One method of designing cable-less accessors is to replace the cable with a sliding electrical brush contact that delivers power and communication to the cable-less accessor. Brush contacts have been previously utilized with electric trains and trolleys as well as elevators. These types of brush designs use self-lubricating composite brushes. The electrical brush contact works by having the composite brush slide on a linear track that is typically made of a copper material, either a raw copper track or exposed copper within a printed circuit board. The electrical connection is made by the brush always staying in contact with the copper track by sliding along its surface. In order to prevent the brush from quickly wearing away during this sliding motion embedded solid lubricants are added to the brush. The nature of this lubrication mechanism requires that the brush must experience some wear to transfer lubricant from the brush to the copper track. This wear creates electrically conductive debris that can build up over the long life experienced by an accessor in a tape library. Additionally, for modularly expandable libraries, gaps or joints between adjacent sections of copper track must be carefully constructed to minimize wear on the brushes. This careful alignment of tracks adds significant cost to the design of the library. Although modular libraries are highly desirable, the presence of a gap between adjoining components leads to undesirable wear on the brushes.
A further issue that arises when composite brushes are used is contamination of the copper track by the lubricant in the brush. The main form of contamination that is seen is a silver sulfide, which appears as tarnish on the copper track surface. The silver sulfide film on the surface of the copper track creates an unacceptably high contact resistance on low current circuits. Although the low current communication signals to the robotic accessor can be modulated on top of the higher current power signals to the accessor, this adds additional cost and design complexity by requiring circuitry to modulate and demodulate these low current signals.
A different type of brush than composite brush is a fiber brush for electrical contacts for slip rings, as shown in prior art FIG. 1. Fiber brushes have advantages of multiple points of contact per brush bundle, no surface lubrication required, long life, low contact force per fiber, low wear rates, low contact resistance, very little debris generation, and a wide range of operating speeds. However, FIG. 1 shows that the fibers of the brush are always touching the round slip ring surface and are tangent to this surface such that the rotating ring can rotate in either direction, clockwise or counter-clockwise. Such a configuration mandates undesirable design characteristics for a tape library application.
What is needed is a low wear, high reliability, and low cost method of transferring electrical power and communication to the robotic accessor. It is therefore a challenge to overcome these, and other, disadvantages.